Surface aerators are some of the hardest-working gearboxes in any plant. They run 24 hours a day, every day, carrying the weight and thrust of the impeller, mounted over the tank, out in the sun and constantly in their own spray. When an aerator gearbox fails, dissolved oxygen drops and the biological process suffers within hours. Most of these failures trace back to the oil: the wrong grade, oil left in too long, or water in the oil.
Use the EP gear oil grade the gearbox maker specifies, usually ISO VG 220 or 320 (CLP type). Change mineral oil at the maker's interval or when an oil test says so, and consider a synthetic PAO gear oil for hot, hard-to-reach or critical units, where it often lasts several times longer. Water is the main enemy: keep breathers dry, keep seals in good shape, and check the oil for a milky look every week.
| Situation | Typical grade (follow the maker) |
|---|---|
| Most helical and bevel-helical aerator gearboxes | ISO VG 220 EP (CLP) |
| Hot climates, heavily loaded or slower units | ISO VG 320 EP |
| Worm gear units (some clarifier and older aerator drives) | Often a heavier grade such as VG 460, or a special worm gear oil, as the maker specifies |
Use an extreme-pressure (EP) oil unless the maker says otherwise; our article on EP vs non-EP gear oil explains why. For more on viscosity choice, see choosing industrial gear oil grades.
Some worm gear units are factory-filled with a polyglycol (PAG) oil. PAG does not mix with mineral or PAO oil, so never top up a PAG-filled unit with anything else.
Gearbox makers give change intervals in hours or years, for mineral and for synthetic oil. Two factors shorten them sharply:
Rather than changing oil only by the calendar, sample it. A simple oil test showing viscosity, water content, acid number and wear metals tells you whether the oil can stay in. Our article on fixed oil change intervals explains the idea.
Water in gear oil reduces film strength, causes rust on bearings and speeds up oxidation. Signs are a milky or cloudy look, a rising oil level, or droplets at the drain plug.
A common alarm level for water in gear oil is a few hundred parts per million. Milky oil means far more than that and needs an oil change and the cause fixed.
PAO synthetic gear oil keeps its viscosity better across temperatures and resists oxidation far longer than mineral oil. It usually makes sense when:
When switching from mineral to PAO, drain the old oil completely while warm and refill; PAO is generally compatible with mineral oil residues and common seals, but check the maker's guidance. Never switch to or from PAG without a full flush.
Look at the sight glass for level and colour, check for leaks under the gearbox and at the shaft seal, look at the breather desiccant colour, feel or measure the housing temperature, and listen for new noise. Write it down. Trends tell you more than single readings.
Aerator motor bearings and nearby pump bearings need grease too. A polyurea grease suits electric motor bearings; for wet bearings exposed to spray, a calcium sulfonate complex grease resists wash-out and rust far better than ordinary lithium grease.
For the full range of lubricants for treatment plants, see our page on water treatment lubricants.
Which oil grade is used in aerator gearboxes?Usually an EP industrial gear oil of ISO VG 220 or 320, as the gearbox maker specifies. Worm gear units may need a heavier or special oil.
How often should aerator gearbox oil be changed?At the maker's interval, shortened for high temperatures and contamination. Oil testing for viscosity, water, acid number and wear metals tells you when a change is really due.
Why does gearbox oil turn milky?Water has got into the oil, usually through the breather or a worn shaft seal. Change the oil and fix the cause.
Can I switch an aerator gearbox to synthetic oil without flushing?From mineral to PAO, drain the old oil completely while warm and refill, following the maker's guidance. A switch to or from PAG oil always needs a full flush.
Operating an STP, ETP or water treatment plant? Send us the aerator and gearbox makes, and our team will recommend the oil grade and a change and sampling routine.